1. Field of the Invention
The invention relates to a method according to the preamble of claim 1 for measuring the water content of a liquid.
2. Description of Background Art
The invention also relates to an apparatus for measuring the water content of a liquid.
U.S. Pat. No. 5,331,287 describes a sensor, wherein interdigitated electrodes (finger electrodes are coated with a conducting polymer. Water contained in the oil hydrates the polymer and thus modifies its conductivity. Also detects possible acids by way of protonation.
U.S. Pat. No. 5,642,098 discloses a ring oscillator circuit, wherein electrical properties of the oil are measured with a number of measurement heads which measure the same parameter.
U.S. Pat. No. 5,644,239 measures the electrical conductivity of a liquid (oil) at two elevated temperatures. The technique may be complemented with a possible optical measurement of oil opacity. A xe2x80x9cfigure of qualityxe2x80x9d may then be computed for the oil from these parameters.
U.S. Pat. No. 5,656,767 describes a sensor system for measuring the change of an electrical parameter value (e.g., capacitance) in oil as a function of time. The same oil at a clean (dry) state may be used as a reference value. The same technique may be varied in multiple ways, e.g., by heating the oil sample.
Conventional techniques are handicapped in many aspects. Common methods for sensing absolute water volume content over the entire range of 0-100% are the measurement of the dielectric coefficient and measurement of IR absorption. Both of these methods have in common that they require a zeroing step of the measurement system, whereby the reading must be reset to zero water content when the sensor is brought to measure an entirely dry (water-free) liquid. This step can be accomplished as a discrete zeroing operation or by using a sample of entirely dry oil in the sensor as a reference.
An additional complication arises therefrom that such a zero setting is typically dependent on the temperature.
Also other factors besides the water content may affect the zero-value with the aging of the liquid.
Methods measuring the absolute water content are favored at high water contents (in the order of several per cent).
At lower water contents, problems generally arise from the marginal detection threshold and offset uncertainty (error of zero setting).
A relative value (aw) measurement method gives information on the water content value in relation to that of a fully saturated situation. However, a conversion to the volume percentage value of absolute water content remains undefined unless a conversion factor for the liquid being measured is known. The aw measurement method is suitable for use at low water content levels (nonsaturated and not emulsified), whereby the measurement has a sufficiently high sensitivity. Moreover, the method is free from zeroing problems.
It is an object of the present invention to overcome the drawbacks of the above described techniques and to provide an entirely novel type of method and apparatus for measuring the water content of a liquid.
The goal of the invention is achieved by way of measuring the water content of the oil/liquid using two different methods simultaneously, whereby the measurement technique is based on an absolute value measurement method complemented with a relative value measurement method.
The method and apparatus of the present invention offer significant benefits.
The combination of an absolute value measurement method with a relative value measurement method (aw-type of measurement) makes it possible to eliminate the need for a zeroing step in the absolute value measurement.
By way of performing a sequence of rapidly repeated measurements at different temperatures, it is also possible to eliminate errors caused by temperature variations.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.